arm64/nterp: Introduce `xCSTMP` and use it.

Introduce callee-save temporary register `xCSTMP` (x28) and
its 32-bit variant `wCSTMP` (w28) even though the latter is
not yet needed.

Change `invoke-interface` to use `xCSTMP` instead of x26 and
clean it up to avoid using the 32-bit variant.

Test: testrunner.py --target --64 --interpreter
Flag: EXEMPT PURE_REFACTOR
Change-Id: I888de6ec53d166a09e486eb4cf4f4dc022d577eb